1. Field of the Invention
The present invention relates to an exposure condition measurement method which is used in the projection of the image of a mask pattern on to a photosensitive substrate via a projection optical system. In particular, the exposure condition measurement method of the present invention is suitable for use in cases where the image of an evaluation pattern on a mask is actually exposed on a photosensitive substrate and the optimal focus position of the projection optical system is obtained from that exposure image.
2. Related Background Art
In the past, in photo lithographic processes for the manufacture of semiconductor elements, liquid crystal display elements, image pickup elements (CCDs, etc.), thin film magnetic heads, etc., a projection exposure apparatus such as a stepper which performs batch exposure of the pattern on a reticle (known as a mask or photomask, etc.) onto each shot area of a wafer (a photosensitive substrate or glass plate, etc.) has often been used. Also, a scanning type projection exposure apparatus (step-and-scan method) is being used. In the step-and-scan method, the transfer exposure of the image of a pattern on a reticle onto each shot area on a wafer is accomplished by the synchronous scanning of the reticle and wafer in a state in which a portion of the pattern on the reticle is projected onto the wafer.
With these types of projection exposure systems, in order to transfer the pattern on the reticle onto the wafer with high precision, it is necessary to align the the position of the surface of the shot area of the wafer with the image-formation plane (the best focus position) of the projection optical system. For this purpose, before the exposure-transfer of the image of the pattern on the reticle onto the wafer, the best focus position of this projection exposure apparatus must be determined. In the past a method of obtaining this best focus position has been used whereby the image of a pattern for measurement on the reticle is actually exposed on the wafer while the wafer is moved in the height direction in specified intervals and the best focus position is obtained by the measurement of that exposed image using a suitable measurement instrument.
In the prior art method of obtaining the best focus position, the projection exposure apparatus is provided with an alignment sensor as a position detection apparatus for detecting the positional deviation between the reticle and wafer. Several different types of alignment sensors may be used to perform this function. For example, an LSA (laser step alignment) system may be used, whereby laser light is projected on alignment marks (in the form of a dotted line) on the wafer, and the position of the alignment mark is detected using the light refracted or scattered by the mark. Also, an FIA (field image alignment) system may be used, whereby illumination with a light having a wide wavelength band width (i.e. a halogen lamp) is performed, and the image data of the alignment mark that has been imaged is measured by picture processing. Finally, an LIA (laser interferometric alignment) system may be used, whereby alignment marks in the form of a diffraction grating on the wafer are illuminated from two directions with laser light having the same frequency or slightly different frequencies, and the two diffracted lights produced are made to interfere, whereby the position of the alignment mark is measured from the phase thereof.
In a method in which the best focus position is obtained based on the above-mentioned actual exposure image, an LSA-type alignment sensor has been used. In this case, a reticle, on which an evaluation pattern is formed, is used. The evaluation pattern contains, for example, multiple diamond-shaped patterns in the measurement direction arranged in a lattice form. In operation, while the position in the direction of the optical-axis of the projection optical system is changed in stages, the length of the exposure image obtained by the actual exposure of the evaluation pattern on the wafer is measured by means of an LSA-type alignment sensor, and the best focus position is detected from this measurement result.
FIG. 9 shows the exposure image of a conventional evaluation pattern on a wafer. Pattern images 51X1 through 51X3 (evaluation patterns for the X-axis) have a long diamond shape in the X-direction and pattern images 51Y1 through 51Y3 (evaluation patterns for the Y-axis) have a long diamond shape in the Y-direction are formed on the wafer. In operation, the closer the surface of the wafer approaches the best focus position, the longer the X-axis pattern images (51X1 through 51X3) become in the measurement direction (X direction) and the farther the position of the wafer is separated from the best focus position (de-focusing), the length of the pattern images in the measurement direction is shortened. Accordingly, the length of the pattern image in the measurement direction is measured by an LSA-type alignment sensor, and based upon the results thereof, the best focus position can be obtained.
Since the method of detecting the best focus position of the projection image system using a conventional LSA-type alignment sensor is a method whereby the length in the measurement direction of diamond-shaped mark images formed on the wafer are compared, the measurement precision is dependent upon the measurement precision of the LSA-type alignment sensor. In recent years, in conjunction with the integration of enhanced "super LSAs," circuit patterns on the reticle have been transferred at higher levels of precision onto the wafer. Nevertheless, in conventional methods using this LSA-type alignment sensor, there still exists the problem that it is difficult to further improve the detection precision of the best focus position.
Also, when diamond-shaped measurement marks are used as shown in FIG. 9 with an alignment sensor other than the generally used LSA-type (i.e. an FIA-type or LIA-type) in the projection exposure apparatus, the measurement of the best focus position is difficult.